The present invention relates to implantable electrical leads generally; and, more specifically, to cardiac pacing leads.
The conductors in cardiac pacing leads occasionally have a tendency to fracture due to repetitive application of stress to the conductor. One way in which this problem has previously been addressed is by reinforcing the lead body in the area in which stress is to be expected, as in U.S. Pat. No. 5,545,203, issued to Doan et al. Reinforcement of the lead body is also disclosed in U.S. Pat. No. 5,591,142, issued to Van Erp et al. It has also been proposed to reinforce the lead body by means of adding a tensile reinforcement as in U.S. Pat. No. 5,231,996 issued to Bardy et al. In this patent, the lead is provided with a non-conductive tensile member such as a polyester cord, which runs the length of the lead body. Other leads having cords or reinforcements running throughout their length are disclosed in U.S. Pat. No. 3,844,292 and U.S. Pat. No. 3,572,344 issued to Bolduc. A third proposal for dealing with the possibility of conductor fracture is to render the portion of the lead body in direct contact with the conductor conductive by addition of carbon or other conductive material, as disclosed in U.S. Pat. No. 4,033,355, issued to Ammundson.
Despite prior efforts, an improved mechanism is needed to prevent failures of the conductive cables carried by lead bodies, wherein the failures are due to compressive forces.
The present invention is directed toward providing a temporary backup for electrical conduction in the event of a conductor failure within a lead. An implantable medical device (IMD) such as lead or catheter having a conductor for conducting an electrical signal includes a safety cable to provide a backup path for electrical current if the cable conductor fails. In one embodiment, the conductor is a cable positioned adjacent to the safety cable. In another embodiment, the IMD includes multiple coaxially configured coils, with a safety cable being positioned between an insulative layer adjacent the inner coil to provide redundancy for the outer coil.
According to one aspect of the invention, the safety cable may have a smaller diameter than the conductor cable. In this instance, the safety cable is designed to add as little as possible to the overall diameter of the lead body, but has enough cross section to adequately carry the electrical current required to temporarily maintain the function of the lead if the conductor cable should fail. The safety cable may be in electrical contact with the conductor along one or more portions of its length. Alternatively, the conductor and safety cable may be electrically insulated from one another except for connection points at the distal and proximal ends of the conductor. The latter embodiment allows for the detection of a failure in the conductor using impedance changes that occur in the lead after the failure occurs.
In one embodiment of the invention, the connections of the safety cable to the conductor need only assure intimate contact between the two cables for non-intermittent conduction of electricity and need not have the same mechanical integrity to withstand tensile loading as the rest of the lead body members. In addition, the safety cable may be assembled into the lead body free of residual tensile stress. In another embodiment, a lead might contain multiple conductors, each being associated with a respective safety cable.